In general, a ground anchor device is used in the field of public works as a construction material for stably maintaining a structure from excessive stress, deformation, displacement, etc. occurring in the ground, by fastening a high strength tension member, such as a PC steel wire, both to the structure and the ground and then applying a prestress to the tension member.
Ground anchor devices can be divided into a tension type anchor device, a compression type anchor device, and a load dispersion type anchor device for dispersing a load in the tension type anchor device and the compression type anchor device, depending upon a supporting type of a mounting ground.
Referring to FIGS. 1 and 2, a tension type anchoring method is a method in which a tension member 104 is inserted into a borehole defined in the ground, a grout 102 is filled in the borehole and a tension is applied to the tension member 104 so that the ground is supported by a frictional force between the ground and the grout 102. The tension type anchoring method suffers from defects in that, since a crack is likely to be created in the grout 102 by the tension applied to the tension member 104 and a progressive failure is likely to occur by a creep due to the concentration of a load, load decrease becomes substantial. Accordingly, in the graph of FIG. 2 showing a skin friction distribution, while a load transition distribution of the load curve 1 representing initial loading is obtained in an initial load application stage, the load curve 3 is resultantly obtained due to creep rupture, etc. with the lapse of time, by which a load decreases. Also, when the tension force is applied to the tension member 104, although an originally designed load curve is anticipated, a load concentration section exceeds the ultimate uplift capacity of a target ground. As a result, a load is decreased as in the load curve 2 and is fixed to the load curve 3. This mainly results from decrease in local frictional force due to load concentration.
Referring to FIGS. 3 and 4, a compression type anchoring method is a method in which PC steel strands coated with polyethylene (PE) are confined to a separate fixing body 106 to generate a compression force in a grout 102. In the compression type anchoring method, since tension is applied to the fixing body 106 via a tension member 104 and the fixing body 106 compresses the grout 102, load decrease due to a creep is reduced when compared to the tension type anchoring method. However, the compression type anchoring method has a drawback in that, since a grout with a high strength should be used, it is difficult to secure a predetermined anchoring force in a relatively soft ground. Also, as can be seen from FIG. 3 showing load changes, load concentration of the compression force applied to the grout 102 occurs on a distal end. Such a load concentration may break the grout 102. Further, in the case where it is necessary to apply a load exceeding an ultimate frictional force, disadvantages are caused in that the diameter of a borehole should be increased or bonding should be implemented with respect to a rock with a high surrounding confinement pressure. In the compression type anchor device, similar to the tension type anchor, load decrease occurs (see FIG. 3 showing load changes), and an abrupt load decrease may occur due to a compressive failure. As can be seen from FIG. 4 as a graph of a skin friction distribution, in the case of a load concentration type anchor, a load transition distribution changes from the load curve 1 to the load curve 3, by which a load decreases.
Referring to FIGS. 5 and 6, a load dispersion type anchoring method is a method in which the disadvantages of the tension type anchoring method and the load concentration type anchoring method are overcome and tension type anchors are dispersed to various portions. In the load dispersion type anchoring method, an extreme load concentration should not be occur in a bonding ground and a grout 102, PC steel strands coated with PE should be used not to confine a free part, and it should be easy to adjust an allowable anchoring force depending upon a ground condition. In the case of a load dispersion tension type anchor device, since a load is applied to a number of tension members 104 in a dispersed manner, an influence on the strength of the grout 102 becomes flimsy and a predetermined anchoring force can be secured even in a relatively soft ground. As a consequence, a high load as in a rock can be exhibited in a general soil ground.
In the load dispersion type anchor, as can be seen from FIGS. 5 and 6 showing a load distribution diagram and a graph of a skin friction distribution, since a load is applied to the ground in a dispersed manner, load decrease becomes very small, and an initial load distribution can be maintained regardless of a time.
The above-mentioned load dispersion type anchor device is disclosed in Korean Utility Model Registration No. 0375568 entitled “Frictional Force and Tension Dispersed Composite Ground Anchor” which was filed and registered in the name of the present applicant. That is to say, the above-mentioned complex ground anchoring method is a method in which, as shown in FIG. 7, fixing anchors a, b and c positioned in a bonded length section are arranged in such a way as to define step-like shapes and respective tension members 221a, 221b and 221c positioned in free length sections are removed from a fixing block 220 after construction is completed. In this anchoring method, a large anchoring force can be secured when compared to the tension type anchor device or the compression type anchor device in which tension members are arranged in line.
The unexplained reference numeral 222 designates spacer members.
In the structure of the complex ground anchor device, as shown in FIG. 8, a cap 225 is threadedly coupled to the fixing block 220, and the fixing anchor a fixed to a squeezing sleeve 231 is connected through the cap 225.